Method of and Arrangement for Feeding Fuel Into a Circulating Fluidized Bed Boiler

ABSTRACT

A method of feeding at least one of light, fine and moist fuel into a furnace of a circulating fluidized bed boiler. Fuel is fed into the furnace through a fuel feed and the fuel is combusted in a turbulent, circulating fluidized bed. A fuel feed area is isolated from the turbulent circulating bed by arranging the fuel feed along at least one channel arranged in a wall of the furnace. Solids of the circulating bed material are introduced onto a first grid section at the bottom of the fuel feed area. The fuel and the solids are mixed and fluidized above the grid section to form a fuel-solids mixture that flows laterally onto a second grid section and a third grid section where the mixture is fluidized. The fluidized bed material is circulated both inside and outside of the furnace. The bed material is separated from the flue gases and returned to the furnace.

This application is a U.S. national stage application of PCT International Application No. PCT/FI2010/050863, filed Oct. 29, 2010, published as International Publication No. WO 2011/058218 A1, and which claims priority from Finnish patent application number 20096170, filed Nov. 10, 2009.

FIELD OF THE INVENTION

The present invention relates to a method of and an arrangement for feeding fuel into a circulating fluidized bed boiler (CFB). The invention is specifically concerned with feeding fine, light and/or moist fuel into the boiler.

BACKGROUND OF THE INVENTION

A circulating fluidized bed boiler generally includes a furnace having a bottom, side walls and a roof, and at least one particle separator connected in flow communication with the upper portion of the furnace. At least some walls of the bottom portion of the furnace may be inclined such that the cross section of the furnace increases upwardly, i.e., the portion of the furnace having the inclined walls may be called a converging bottom portion. In practice, all of the walls and the roof of the boiler and the separator comprise water tubes to collect heat from the furnace. The bottom of the furnace is provided with a grid for introducing combustion or suspending or fluidizing gas, called primary air, into the furnace, and for removing ash and other debris from the furnace. The side walls of the furnace are provided with means for introducing fuel into the furnace, as well as means for introducing secondary air into the furnace. The furnace is also equipped with means for feeding inert bed material that is normally sand into the furnace.

The particle separator separates solid particles from a flue gas-solid particles suspension entering the separator from the upper portion of the furnace. The flue gases are taken out for further treatment from the separator, and separated solids are recycled back to the lower portion of the furnace via a recycling conduit that includes a sealing device, such as a loopseal, the purpose of which is to prevent gas from flowing from the furnace to the separator via the recycling conduit. Thus, at least a further opening in the furnace wall is needed for the solids introduction. This solids circulation is called an external circulation. In addition to vertical upflow of the flue gas-solid particles suspension in the furnace finally entering into the separator inlet, there is a vertical downflow of particles near the furnace walls. This solids circulation is called an internal circulation.

Very often, in connection with the internal or the external circulation of solid material, or both, at least one fluidized bed heat exchange chamber has been arranged to transfer heat from the bed of fluidized particulate solids to a heat transfer medium. The heat exchange chamber may be located inside the furnace of the circulating fluidized bed boiler adjacent to at least one of the furnace walls. A preferred location for the heat exchange chamber (or chambers) is integrated with the inclined wall (or walls). The fluidized bed heat exchanger may also be arranged in the external circulation, so that the solids leaving the solids separator are discharged into the heat exchange chamber on their way back to the furnace (see, for example, the prior art shown in FIG. 1). The interior of the heat exchange chamber is provided with heat exchange means for heat transfer from the solid material to the heat transfer medium flowing inside the heat exchange means.

Normally, fuel is introduced into the furnace via one or more openings in a planar, either vertical or inclined wall of the furnace. The fuel is, depending on the type of fuel, proportioned in the furnace either as a fuel-air suspension, i.e., pneumatically, or by means of a screw feeder or some other mechanical feed means. Normally, the fuel opening is (openings are) located in the (converging) bottom portion of the furnace walls.

The solids entering the furnace from the external circulation, i.e., directly from the separator or via a fluidized bed heat exchanger, are also introduced into the furnace via one or more openings in the planar furnace wall.

Light, fine and/or moist fuels, such as, for example, fine coal powder or peat or sawdust or fine lignite, are problematic in two different aspects. Light, small density and fine particle size fuels are easily entrained with the fluidized gas, and rapidly rise upwards so that the combustion process starts a few meters above the grid, whereby only a small amount of fuel, not sufficient to maintain the bed temperature at a sufficient level, is combusted in the lower bed area, with most of the fuel being combusted higher up in the furnace. This may result, especially in low load conditions, in too low a bed temperature, and a higher temperature in the upper portion of the furnace, which, again, may lead to problems in emissions and in the load change rate of the boiler.

In a similar manner, the use of moist fuel may result in similar problems, but for a somewhat different reason. Though the moist fuel may not be too light, the drying thereof requires some time, such that the fuel is again, while it is drying and not yet capable of igniting, lifted by the fluidizing gas in the upper portion of the furnace. When the fuel is finally dry enough, ignites and is finally combusted, there may not be enough combustible fuel in the lower bed area, whereby the bed temperature may, again, be low, and result in the problems already discussed above.

SUMMARY OF THE INVENTION

An object of the invention, therefore, is to ensure, for the light, fine and/or moist fuel, sufficient residence time for drying and/or combustion in the lower bed area of the furnace, for optimizing the entire combustion process. It has been understood that the residence time is dependent on at least the following factors. First, the better the fuel feed is isolated from the turbulent interior of the furnace, the better the residence time can be adjusted. In other words, if the fuel can be taken down to the grid area substantially without the fuel being mixed with the turbulent bed material flowing up in the furnace, the fuel has time to dry. Also, even if the fuel may be dry, taking the fuel down to the grid area ensures that the fine and light fuel is not taken too quickly in the upper portion of the furnace, but ignites and combusts in the lower portion thereof. Several methods and arrangements for isolating the fuel feed from the turbulent bed have been discussed in a copending U.S. patent application Ser. No. 13/503,957, filed Apr. 25, 2012, of Foster Wheeler Energy Corporation. Second, the more controlled the manner in which the fuel is handled down at the grid area, the better the combustion of the light and fine fuel in the lower portion of the furnace is controlled. According to the present invention, the residence time of the fuel in the lower bed area is increased by retarding the movement of the fuel to the upper bed area, or even to the circulating bed area, by means of controlling the substantially lateral movement of the fuel at the grid area, and by making special arrangements therefor at the grid area of the furnace.

Several prior art documents discuss the handling of the fuel and/or bed material at the grid area.

It is, for instance, known to make the bed move in a desired direction by arranging the grid at an incline, and/or by fluidizing the bed in a certain manner, so that the fluidized bed flows like fluid along the grid. For instance, Japanese patent documents JP-A-2000-65327 and JP-1-63-73091, U.S. Pat. No. 5,138,982 and No. 4,270,468 discuss such arrangements. U.S. Pat. No. 5,138,982, for example, discusses a fluidized bed boiler in which the bottom of the furnace is constructed of a grid, which is adapted to inject fluidizing air upwardly under a mass flow that is greater at a first side than that at a second side. The mass flow at the first side is capable of transporting bed material up in the furnace, whereas the mass flow at the second side merely fluidizes the bed material. The furnace also comprises an inclined wall provided above the first side of the grid where the mass flow is greater, so as to interfere with the upward flow of the fluidizing air and fluidized bed material, and thereby to deflect the air and bed material towards a portion above the second side of the grid where the mass flow is less. A moving bed is formed above the portion of the grid where the injected mass flow is less, so that fluidized bed material descends and diffuses within the moving bed, and a circulating fluidized bed is formed above the grid.

U.S. Pat. No. 7,240,639 B2 discusses an inclined grid, but also, a so-called step grid structure where the grid is provided with nozzles that direct, first, the fluidizing gas, and thereby, also the bed material, in a desired direction. The step grid is arranged between a fluidized bed heat exchanger and the actual grid area of the furnace, which are positioned substantially at the same horizontal level. The step grid is used in the above-mentioned patent for preventing large pieces prevailing in the bed material from entering the heat exchanger, by directing a fluidizing gas flow away from the heat exchanger.

European patent document EP-A1-0 124 636 discusses a fluidized bed boiler having a bottom part divided into two chambers by means of a vertical wall that is arranged to leave a gap between the bottom of the furnace and the wall lower end. A first chamber is used for receiving, on the one hand, recirculated fly ash and fine fuel particulates, and, on the other hand, bed material that has been raised upwards by the fluidizing gas, and, after having reached the level of the upper edge of the vertical wall, has flown over the edge into the first chamber. The second chamber is used for creating the fluidized bed. The bottom of the first chamber is formed of a gas tight water tube wall, whereas the bottom of the second chamber is formed of an ordinary grid having gas nozzles for introducing fluidizing gas in the furnace. The outside wall of the first chamber opposite to the separating wall and the above-mentioned gap is provided with gas nozzles, the purpose of which is to pneumatically transport the fly ash and fuel particles through the gap into the second chamber.

The patent documents mentioned above, however, do not discuss a controlled treatment of fine and light, possibly, moist fuels at the grid area, in view of their controlled introduction in the fluidized circulating bed.

A further object of the present invention is to suggest a novel grid structure and a method for introducing light, fine and/or moist fuel in the fluidized circulating bed.

A still further object of the invention is to suggest a method and a grid structure where the fuel is introduced down to the grid area, so that the fuel is isolated from the turbulent bed material, and the grid area is designed to delay the feeding of the bed material-fuel mixture to the circulating bed until the fuel is delivered to a substantial area of the grid.

Other features of the method and the apparatus of the present invention can be seen in the appended claims.

By means of the present invention, at least some problems relating to the feeding and combustion of fine, light and/or moist fuel in a circulating fluidized bed boiler have been minimized by means of a simple and an effective means of feeding fuel into a furnace of a circulating fluidized bed boiler. For instance, there is no need to design and to build external drying chambers, known from the prior art, to dry moist fuel. Also, the need of mixing light, powdery fuel with hot bed material in a separate mixing chamber has been obviated.

The present invention makes it possible to keep the bed temperature higher, even in the lower portions of the furnace, when light, fine and/or moist fuel is combusted. This is particularly true with lower boiler loads, when the bed temperature tends to also decrease for natural reasons. Compared to prior art arrangements, the present invention, when taken into use, provides that:

-   -   the moist fuel starts drying sooner,     -   the dried fuel ignites and combusts sooner, and     -   a higher portion of the light and fine fuel is taken down to the         bed, whereby the bed temperature gets higher or remains at an         acceptable level, even in low load conditions.

A further advantage in using the present invention is that the fuel feed may be arranged higher on the wall of the furnace, whereby the counter pressure is smaller. This advantage is not necessarily limited to light and fine fuels, but may also be applied to all types of fuels.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the method and the arrangement of the present invention will be explained in more detail with reference to the following drawings.

FIG. 1 is a schematic representation of a circulating fluidized bed boiler of the prior art.

FIG. 2 is a schematic representation of a first preferred embodiment of the present invention.

FIG. 3 is a schematic representation of a second preferred embodiment of the present invention.

FIG. 4 is a schematic representation of a third preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates a circulating fluidized bed boiler of the prior art. The boiler 10 comprises a furnace 12, with substantially vertical walls 32, a discharge passage 14 in the upper end of the furnace 12 for taking the flue gas and solid particles suspended thereby to a solids separator 16, a passage 18 arranged in the upper end of the solids separator 16 for the removal of the cleaned exhaust gas from the solids separator 16, a recirculation conduit 20 at the lower end of the solids separator 16 for returning at least a portion of the separated solids back to the lower portion of the furnace 12, fuel feed means 22 arranged at a side wall 32 of the furnace 12, and means 24 and 26 for introducing primary and secondary air, respectively, arranged at the lower portion of the furnace 12. The fuel feed means 22 may include a screw feeder, a drop let, or a pneumatic feeder, just to name a few alternatives. The primary air 24 is the primary combustion gas that is also used to fluidize the bed material, and is thus fed into the furnace 12 through the grid 60 arranged at the bottom of the furnace 12. The secondary gas 26 is introduced into the furnace 12 through the side wall 32 thereof slightly above the grid 60. A gas lock 28 has been arranged in the recirculation conduit 20 for preventing the gas from flowing from the furnace 12 via the recirculation conduit 20 into the solids separator 16. Here, the recirculation conduit 20 is further provided with a fluidized bed heat exchange member 30 for collecting heat from the recirculating solids to a heat transfer medium. The side walls 32 of the boiler 10, as well as the ones of the solids separator 16, usually comprise water tubes, so that water acts as the heat transfer medium.

FIG. 2 schematically illustrates a first preferred embodiment of the present invention showing such a portion of a side wall of the bottom portion of a furnace 12 that has the fuel feed arranged in connection therewith. The wall 32′, most often inclined, of the bottom portion of the furnace 12 is provided with a channel 42 extending in a substantially vertical direction, and being formed of a substantially vertical bottom wall 43, and two substantially vertical side walls 46. The bottom wall 43 is near its lower end provided with an opening 40 for introducing fuel from fuel feed means 22 (FIG. 1) into the furnace 12. The area of the channel 42 between the fuel feed opening 40 and the lower end of the channel 42 is called the fuel feed area. The bottom of the furnace 12 is formed of a grid (shown by reference numeral 60 in the prior art shown in FIG. 1), which is, in accordance with the present invention, divided in three functionally different sections 62, 64, and 66, respectively. Preferably, each grid section has its own air plenum below the grid section. At its lower end, the channel 42 terminates to a first grid section 62, which is designed to provide the lower end area of the channel 42, i.e., the bottom of the fuel feed area, flow conditions different from the rest of the grid area. A second grid section 64 is arranged at the bottom of the furnace between the first grid section 62 and a third grid section 66 of the furnace. The second grid section 64 is also separated, at least functionally, from the other grid sections 62 and 66, such that it has a function of its own. The third grid section 66 works in a manner similar to the prior art grids, i.e., fluidizing the bed with high velocity primary air and participating in the formation of the circulating bed.

In its basic form, the fuel feed arrangement of the present invention works such that fuel, when it is introduced from the opening 40 into the channel 42, and even if the fuel is light, and fine and/or wet, flows down in the fuel feed area onto the first grid section 62. This kind of downflow of the fuel is ensured by isolating the fuel feed area from the turbulent bed. In other words, the channel 42 is deep (in the horizontal direction) and narrow enough to prevent the turbulence prevailing in the furnace 12 from reaching the fuel feed area in such a strength that the fuel would be efficiently mixed with the turbulent bed material, and carried away therewith. The gas flow from beneath the bottom of the grid section 62 fluidizes, and mixes the fuel and solids collected in the lower end of the channel 42, such that the fuel and solids flow from above is able to push the fuel-solids mixture in a substantially lateral direction out of the fuel feed area and the first grid section 62, to the second grid section 64. This substantially lateral movement of the fuel-solids mixture may be, but is not necessarily, enhanced by arranging the first grid section 62 to slope towards the second grid section 64, for instance, by forming a so-called step grid, discussed in more detail in U.S. Pat. No. 7,240,639 B2 and/or by arranging directional air nozzles in the first grid section. Directional air nozzles are nozzles that direct the air jet in a desired direction, in this case, from the lower end of the channel 42, i.e., from the first grid section 62 towards the second grid section 64.

The velocity of the fluidizing air in the first grid section 62 has to be adjusted carefully, so that the escape of the fuel and solids to the turbulent area of the furnace 12 is not facilitated by too high a velocity, but the velocity is maintained low enough for just fluidizing the fuel and solids sufficiently, for mixing them and for making them flow like a fluid to the second grid section 64. The velocity of the fluidizing air is mainly dependent on the particle size of the solids entering the first grid section. The performed tests have shown that the air velocity in the first grid section 62 should be kept between 10 and 20 Umf, where Umf is the minimum fluidization velocity of the particular particle size. For example, if the bed material entering the channel 42 and, later on, the first grid section 62, is coming mainly from a fluidized bed heat exchanger, the average particle size is, based on experience, between 0.2 and 0.4 mm. The Umf for such particles is about 1 to about 6 cm/s, whereby the maximum velocity in the first grid section 62, with these preconditions, is 1.2 m/s. In practical applications, the air velocity should be somewhere between about 0.1 and about 1.2 m/s, preferably, below 1 m/s, somewhat depending also on the particle size of the solids entering the channel from another source (or other sources), and of the fuel.

The second grid section 64 is designed to maintain the fluidized state of the fuel solids mixture and to move the fuel-solids mixture substantially laterally along the grid to the third grid section 66. The substantially lateral feeding of the fuel-solid mixture is accomplished by using either a step-grid structure discussed in more detail in U.S. Pat. No. 7,240,639 B2, or directional nozzles that jet a stream of air to a desired direction. To make the fuel-solids mixture move in the desired direction, the air jets must have a certain velocity. The performed tests have shown that the air velocity in the second grid section 64 should preferably be on the order of about 40 to about 50 Umf, from about 0.4 to about 3.0 m/s, more preferably, below 2.5 m/s, and most preferably, below 2 m/s, in accordance with the above example, where the average particle size was between 0.2 and 0.44 mm. In the third grid section 66, the velocity of the fuel-solids mixture is raised above the terminal velocity (somewhere in the range of 5 to 10 m/s), and the normal operation of the circulating fluidized bed boiler is started.

In accordance with a second preferred embodiment of the present invention, schematically illustrated in FIG. 3, the bottom of the furnace 12 has been provided with a pair of partition walls 68 between the second grid section 64 and the third grid section 66. An exemplary situation when the walls are considered necessary is a boiler bottom portion structure where an opening 72 has been arranged in the side wall 32′ of the furnace 12, close to the bottom thereof, to take a portion of the turbulent bed material to a fluidized bed heat exchanger 44 arranged behind the inclined wall 32′. In this case, the purpose of the walls 68 is to prevent fuel from flowing directly from the feed to the heat exchanger and combusting there. Each wall 68 is located such that the first end of the wall 68 lies against a lower portion of the inclined wall 32′ and the opposite, second end of the wall 68 terminates at a distance from the inclined wall 32′. The distance may be the same as the lateral extension of the second grid section 64 from the wall 32′, or it may be either shorter or, in some cases, also longer. The pair of walls 68 leave between their second ends a free passage for the fuel-solids mixture to flow towards the center of the furnace bottom. The overall dimensions of the walls 68 are dependent on the size of the boiler 10 and on the reason the walls are arranged there. Also, flow guide arrangements other than walls 68 may be used to direct the fuel-solids mixture to a desired location at the third grid section 66. Naturally, it is also possible that there is only one wall 68 at a side of the second grid section 64, if there is not a need for another wall at the other side of the second grid section 64. The walls 68 may be made, for example, of refractory material or of finned water tubes.

FIG. 4 illustrates a third preferred embodiment of the present invention. Here, a fluidized bed heat exchange chamber 44 has been arranged in flow communication with a side wall 46 of the above-mentioned substantially vertical channel 42. The fluidized bed heat exchange chamber 44 receives solids from the internal circulation, via at least one opening 48 arranged thereabove in the wall 32′ of the furnace. Reference numeral 54 shows a further option to receive solids, i.e., an opening via which solids from the external circulation also may be introduced into the heat exchanger 44. Additionally, solids could be taken to the heat exchanger 44 from the lower portion of the bed, as shown by the opening 70 in FIG. 3. The solids entering the fluidized bed heat exchange chamber 44 are fluidized by means of an air current through the bottom 50 of the chamber 44. At the side of the chamber 44, in fact, between the fluidized bed heat exchange chamber 44 and the substantially vertical channel 42, there is a so-called lift-leg 52 connecting the heat exchange chamber 44 to the substantially vertical channel 42. The lift-leg 52 is a small chamber having at a lower end of its side wall facing the heat exchange chamber 44, an opening for allowing the solids flow in the chamber 44, and at an upper end of the opposite side wall, an opening for allowing the solids flow out of the lift-leg chamber 52 to the substantially vertical channel 42. Thus, both the internal circulation flowing down the substantially vertical channel 42 and the solids flow from the heat exchange chamber 44 via the lift-leg 52 mix with the fuel and force the fuel downwards to the lower end of the substantially vertical channel 42, i.e., on the first grid section 62.

Here, it has to be understood that there may be fluidized bed heat exchange chambers 44 on both sides of the substantially vertical channel 42. Also, the position of the fluidized bed heat exchange chambers 44 may be, in relation to the fuel feed opening 40, either higher or lower than shown in FIG. 4. However, the outflow opening of the heat exchange chamber 44 is preferably positioned such that the solids flowing into the channel 42 therefrom flow on top of the fuel entering the channel 42 from the opening 40. But, in case it is considered important, for example, to minimize the counter pressure in the fuel feed opening, the fuel feed opening may be arranged higher in the channel 42 bottom surface, whereby the solids flowing out of the heat exchanger 44 can no longer be used for isolating the fuel feed from the circulating bed. In this case, however, it is still the internal circulation that can be used for the isolation purpose.

It also has to be understood that the substantially vertical channel 42 offers for the solids from the internal circulation a somewhat more peaceful passage flow down towards the grid area than the mere planar surface of the inclined wall 32′. Thus, it is clear that such a channel 42 tends to collect more solids from the internal circulation than a corresponding area on the inclined planar wall 32′. A reason for this is that most of the time, solid particles enter between the channel side walls 46 and meet a less turbulent area, and start to sink towards the bottom of the furnace 12. Thus, the channel 42 itself, through its side walls 46, may be positioned in a vertical plane to collect solids from the internal circulation on top of the fuel feed. This solids collection tendency may be increased, if such a function is desired, by inclining the upper ends of the side wall or walls 46 of the substantially vertical channel 42 outwards, such that the side walls 46 collect internal circulation from a wider area than that shown in FIG. 4. It is also possible to arrange, on the furnace wall, either above or to the sides of the substantially vertical channel, inclined guide plates, preferably made of a refractory material, to collect internal circulation into the channel 42. Likewise, it is possible to arrange a further opening above the fuel feed opening 40, to introduce solids from the external circulation into the substantially vertical channel 42 to be fed on top of the fed fuel.

As an additional feature, relating to the fuel feed, the channel 42 may be provided with a guide plate arranged above the fuel feed opening 40 and positioned to guide, on the one hand, the fuel feed down towards the first grid section 62, and on the other hand, to spread the solids flow on top of the fuel flow entering the furnace 12. Thus, the solids, when forming a kind of a curtain between the fuel and the turbulent bed, assist in isolating the fuel from the turbulent bed material, in addition to the position of the fuel feed deep in the channel, and ensure that fuel reaches the first grid section 62, and is not mixed with the turbulent bed material.

In view of the description above, it has to be understood that only a few most preferred embodiments of the present invention have been discussed. Thus, it is obvious that the invention is not limited only to the embodiments discussed above, but that it can be modified in many ways within the scope of the appended claims. It also has to be understood that features of a specific embodiment of the invention may be applied in connection with features of other embodiments, within the basic idea of the present invention, or that features from different embodiments may be combined to result in a working and technically feasible construction. It is, thus, clear that the above specification, though discussing only one fuel feed opening or one substantially vertical channel, does not by any means limit the number of feed openings or channels of a working circulating fluidized bed boiler, but, rather, the number of various units may freely change in accordance with the requirements of the circulating fluidized bed boiler 10. 

1-23. (canceled)
 24. A method of feeding at least one of light, fine and moist fuel into a furance of a circulating fluidized bed boiler, the method comprising: feeding a flow of fuel into a furnace of a circulating fluidized bed boiler, through a fuel feed, the fuel comprising at least one of light, fine and moist fuel; mixing the fuel with bed material in the furnace to form a turbulent circulating bed; isolating a fuel feed area from the turbulent circulating bed by arranging the fuel feed along at least one channel arranged in a wall of the furnace; introducing solids of the circulating bed material and fuel onto a first grid section at the bottom of the fuel feed area; mixing and fluidizing the fuel and the solids above the first grid section to form a fuel-solids mixture; allowing the fuel-solids mixture to flow substantially laterally onto a second grid section; feeding the fuel-solids mixture substantially laterally along the second grid section to a third grid section where the mixture is efficiently fluidized, and the circulating fluidized bed is created; combusting the fuel in the presence of the fluidized bed material in the furnace and forming flue gases; circulating the fluidized bed material both inside the furnace, in an internal circulation, where bed material returns along walls of the furnace down to the bottom of the furnace, and outside the furnace, in an external circulation, at least via a solids separator arranged in flow communication with the furnace; separating bed material from the flue gases in the separator; removing the flue gases from the separator for further treatment; and returning the separated bed material to the furnace.
 25. The method as recited in claim 24, further comprising isolating the fuel feed area from the turbulent circulating bed by introducing solids from one of (i) the internal circulation flowing down along the furnace walls, (ii) a discharge of solids from a fluidized bed heat exchange chamber, and (iii) the solids returning from the solids separator on top of the fuel feed, such that the solids form a curtain between the fuel and the turbulent circulating bed.
 26. The method as recited in claim 24, further comprising fluidizing and mixing the fuel and the solids above the first grid section by using an air velocity less than about 1.0 m/s.
 27. The method as recited in claim 24, further comprising fluidizing and mixing the fuel and the solids above the first grid section by using an air velocity of between about 0.1 and about 1.2 m/s.
 28. The method as recited in claim 27, further comprising assisting the movement of the fuel-solids mixture to the second grid section by one of (i) at least sloping the first grid section towards the second grid section and (ii) arranging directional air nozzles at the first grid section.
 29. The method as recited in claim 24, further comprising using one of directional nozzles and a step grid at the second grid section for spreading the fuel-solids mixture to the third grid section.
 30. The method as recited in claim 29, further comprising using an air velocity of between about 0.4 and about 3 m/s in spreading the fuel-solids mixture to the third grid section.
 31. The method as recited in claim 29, further comprising using an air velocity of less than about 2.5 m/s in spreading the fuel-solids mixture to the third grid section.
 32. The method as recited in claim 29, further comprising using an air velocity of less than about 2.0 m/s in spreading the fuel-solids mixture to the third grid section.
 33. The method as recited in claim 24, further comprising using flow guide arrangements for spreading the fuel-solids mixture from the second grid section to desired locations at the third grid section.
 34. An arrangement for feeding at least one of light, fine and moist fuel into a circulating fluidized bed boiler, the boiler comprising a furnace delimited at least by a grid at the bottom of the boiler, side walls, and a roof, a solids separator arranged in flow communication with an upper portion of the furnace, an air feed for providing the furnace with primary and secondary air, a return for returning the solids separated in the separator from the flue gases into the furnace, and a fuel feed for feeding a fuel flow into the furnace, the arrangement comprising: a fuel feed area isolated from the rest of the furnace by at least one channel arranged in a side wall of the furnace, for increasing the residence of time of the fuel in the furnace; and a grid divided into three functionally different sections, a first grid section, a second grid section, and a third grid section.
 35. The arrangement as recited in claim 34, wherein the fuel feed area, being isolated from the rest of the furnace, is a portion of a substantially vertically oriented channel arranged in a side wall of the furnace, the channel having side walls, and a substantially vertical bottom wall comprising a fuel feed opening, the first grid section forming the bottom of the fuel feed area.
 36. The arrangement as recited in claim 34, wherein the channel is arranged in flow communication with one of (i) a fluidized bed heat exchange chamber, (ii) the internal circulation and (iii) the solids separator.
 37. The arrangement as recited in claim 34, wherein the fuel feed area is arranged in flow communication with the second grid section to provide flow conditions that are different from those of the first grid section.
 38. The arrangement as recited in claim 34, wherein at least one of the first grid section slopes towards the second grid section and is provided with directional air nozzles for assisting in transferring the fuel-solids mixture to the second grid section.
 39. The arrangement as recited in claim 34, wherein the second grid section is arranged in flow communication with the third grid section, the third grid section having flow conditions that are different from those of the first grid section and the second grid section.
 40. The arrangement as recited in claim 34, wherein the second grid section is provided with guide arrangements for spreading the fuel-solids mixture to desired locations at the third grid section.
 41. The arrangement as recited in claim 40, wherein the guide arrangements at the second grid section are one of (i) direction nozzles and (ii) a step grid for assisting in transferring the fuel-solids mixture to a desired location at the third grid section. 